A photorefractive recording or storage medium or material functions by means of a photorefractive effect in the material. Typically information is recorded or stored in such a medium, using this photorefractive effect, in the form of a hologram. As such, photorefractive recording or storage media may be referred to as holographic storage media. The hologram is generated by incident electro-magnetic radiation, for example visible or infrared radiation, typically laser radiation, of a particular wavelength or frequency.
The photorefractive effect is based on a sequence of steps that include photoionization of localized trapping centers in the material by the incident radiation, and charge transport of resulting free carriers through the material by drift and diffusion. The trapping centers may originate from intrinsic defects or dopant impurities in the material. The free carriers are re-trapped so as to produce a space-charge electric field pattern along with a corresponding refractive index modulation that arises via an electro-optic effect. A detailed account of this photorefractive effect is provided in Photorefractive Materials and Their Applications I, (Springer-Verlag, 1989), ch. 1. Conventionally, the photorefractive effect utilizes single-photon or single-wavelength excitation of the material to achieve photoionization of the charge trapping centers.
For storage applications, promising single-photon photorefractive materials are based on a class of materials known as ferroelectric oxides. These materials are metal oxide host materials including intrinsic defects or, preferably dopants. Some of the earliest ferroelectric oxide materials include nominally undoped lithium niobate (LiNbO.sub.3); iron-doped (Fe-doped) or manganese-doped (Mn-doped) LiNbO.sub.3 ; Fe-doped lithium tantalate (LiTaO.sub.3); iridium-doped (Ir-doped) or uranium-doped (U-doped) LiNbO.sub.3 ; and Fe-doped or silicon-doped (Si-doped) LiNbO.sub.3 or LiTaO.sub.3. These materials are described respectively in U.S. Pat. No. 3,544,189, U.S. Pat. No. 3,915,549, U.S. Pat. No. 3,997,350, and U.S. Pat. No. 4,052,119. More recently, other single-photon recording materials such as cerium doped (Ce-doped) strontium barium niobate Sr.sub.x Ba.sub.1-x Nb.sub.2 O.sub.6 and undoped or rhodium-doped (Rh-doped) barium titanate (BaTiO.sub.3) have been developed.
Typically, holographically stored information is read from the holographic storage medium using radiation of the same wavelength as that used to record the information. In a single-photon photorefractive recording medium this will usually result in erasure of the recorded hologram during readout, unless some type of fixing process, such as thermal ionic fixing, is employed following recording. This is a significant disadvantage of single-photon holographic recording media. In addition, most of the ferroelectric oxide photorefractive materials are not sensitive in the near infrared.
For holographic storage applications, the above discussed disadvantages of single photon recording media may be overcome by using media which take advantage of a multiphoton photorefractive effect. Multiphoton absorption or excitation is a well-known process in which two or more photons of the same or different energies (wavelength or frequency) are absorbed by a material, thereby placing the material in a higher energy configuration. With regard to multiphoton photorefractive recording, the photons involved in the multiphoton excitation step are ideally of at least two different wavelengths.
In general, depending on the properties of the host/dopant combination providing the multiphoton absorbing material, the higher energy states of a dopant may or may not be strongly coupled to a particular host in such a way that charge transfer can occur between the dopant and the host. However, the photorefractive effect, whether arising from a single-photon or multiphoton absorption process, requires a charge transfer to take place during recording and erasure; the charge transfer is also referred to as photoionization. This means that at least one dopant species must act as a charge trapping center for electrons or holes. A portion of the trapping centers are filled while the remainder are empty. Holographic recording and erasure then occur through light-induced charge redistribution among the trapping centers during photoexcitation.
Early efforts to pursue holographic recording applications based on the two-photon photorefractive effect were hindered by a lack of a suitably efficient recording material, i.e., host/dopant combination. Initial work by von der Linde et al. (Appl. Phys. Lett. 25, p. 155, 1974) centered on LiNbO.sub.3 doped with transition metal ions Fe.sup.3+, Fe.sup.2+, and Cu.sup.2+. Experimental results, using very high peak intensity picosecond pulses in the range of 1.times.10.sup.8 to 5.times.10.sup.8 Watts/cm.sup.2 (W/cm.sup.2), indicated that the dopants had little or no effect on observed two-photon absorption and corresponding hologram formation. The high-intensity two-photon recording process was attributed to an intrinsic, but otherwise unspecified, property of the LiNbO.sub.3 host crystal itself. Intrinsic multiphoton absorption and electron excitation from the "homogeneous host material rather than from a dopant" is also taught in U.S. Pat. No. 3,922,061. Two-step photorefractive recording at relatively high intensity, for example about 10.sup.9 W/cm.sup.2 has also been demonstrated in Fe-doped LiTaO.sub.3 (H. Vormann and E. Kratzig, Solid State Commun. 49, p. 843, 1984).
From an applications point of view, the utility of multiphoton holographic recording increases dramatically as the optical intensity requirements decrease, for the simple reason that low-intensity lasers are more readily available, less expensive, and generally more compact than high-intensity laser systems.
Two-photon recording at reduced intensity has been demonstrated in chromium-doped Cr.sup.3+ -doped lithium niobate LiNbO.sub.3 and LiTaO.sub.3 (D. von der Linde et al., J. Appl. Phys. 47, p. 217, 1976). However, the Cr.sup.3+ -doped material was found to have a dark storage time of only about 20 hours, which is not considered useful for most storage applications.
It is believed that less-than-ideal results in multiphoton recording experiments discussed above are due in large part to relatively short lifetimes of intermediate excited states of dopant ions, for example, less than about five-hundred nanoseconds (500 ns) for Cr.sup.3+ -doped LiNbO.sub.3. It is believed that substantially improved multiphoton recording is to be expected with doping schemes that provide longer-lived intermediate excited states.